The genus Malus Mill. of the Maloideae of the Rosaceae consists of
about 35 recognized species, which are distributed in temperate region of the
Northen Hemisphere including Asia, Europe and North America. Redher's
classification divided the genus Malus into 6 series, 5 sections, of which 5 series,
3 section, including 27 species, occur in China. The chromosomal counts on 22
Chinese species were reported previously (Chen et al., 1986; Liang, 1987).
However, no comprehensive karyotype analyses have been made for most species
reported. Analyzed in this paper are karyotypes of 36 forms, 22 species, and
synapsis configuration of 6 polyploid forms in Chinese Malus to explore the
relationships among species and provide cytological evidence for delimitation of
species in Malus. Chromosome classification follows Li and Chen (1985). The
materials used are listed in Table 1 and the vouchers are deposited in our depart-
ment. The cytological results are shown as follows (see the data in Table 1,
Plates 1-- 8 and Fig. 1).
1 Sect. Malus
(1) Ser. 1 Baccata The series consists of nine species endemic to China, 7 of
which were cytologically observed: i.e. Malus baccata Borkh. 2n=34; Sooo1-3
of M. baccata Borkh. 2n= 34; M . mandshurica Kom. 2n= 34; M. rockii Rehd.
2n=34; Sooo 3-2 of M. rockii Rehd. 2n=51; M. sikkimensis Koehne.
2n=34; M. hupehensis Rehd. 2n=51; Sooo4-4 of M. hupehensis Rehd.
2n=51; M. halliana Koehne. 2n=34 and M. xiaojinensis Cheng et Jiang
2n=68. The karyotypes consist of sm (8- 9 homologous chromosomes) and
m (6- 7 homologous chromosomes) as well as st (1-2 homologous
chromosomes). The chromosomes range in relative length from 9.84 to 3.74,
with the ratio of the longest to the shortest from 2.28 to 2.03 and the mean ratio
arms from 2.08 to 1.86. All the karyotypes belong to Stebbins's 3B except 2B of
M. Xiaojinensis. Pollen mother cell (PMC) of M. hupehensis contains
17I+17II, and the average synapsis configuration per cell is 2n=3x=51=17.
70I+ 15. 87II+ 0.52III, while PMC meiosis of M. xiaojinensis shows the synapsis
configufations of 2n=4x= 68=4II+ 15IV. The latter has a remarkable and con-
stant deleted chromosome, which is also the smallest in the karyotype.
(2) Ser. 2 Pumilae. The series consists of six species endemic to China (Yu,
1979), all of which were cytologically examined, with the results: Sooo 9-8 of
M. pumila Mill. 2n=34; Sooo 9-12 of M. pumila Mill. 2n=34; Sooo 9-7
of M. sp. 2n=34; M. asiatica Nakai. 2n=68; M. prunifolia Borkh.
2n=34; Soo 12-2 of M. prunifolia Borkh. 2n=34; M. spectabilis Borkh.
2n=51; M. micromalus Makino. 2n=34 and M. sieversii Roem. 2n=34. The
karyotypes consist of sm(8-9 homologous chromosomes) and m (6- 7
homologous chromosomes) as well as st (1-2 homologous chromosomes). The
chromosomes range in relative length from 10.77 to 3.42, with the ratios of the
longest to the shortest from 2.95 to 2.04, and the mean arm ratios from 2.10
to 1.93. Therefore, all the karyotypes belong to 3B. Among more than 20 differ-
ent forms of M. sieversii, there is only one ploidy level (2n=2x=34). PMC
meiosis of M. spectabilis shows the synapsis configurations 2n=3x=51 =5I +
5II+ 12III.
2 Sect. II Sorbomalus
(3) Ser. 3 Sieboldianae. This series has only one species endemic to China,
M. sieboldii Rehd.: One triploid from Zunyi, Guizhou Province has
2n=3x=51=24m+24sm+3st. The chromosomes range in relative length
8.74-- 4.17, with the ratio of the longest to the shortest 2.08, and the mean arm
ratio 1.80. The karyotype, therfore, belongs to 2B. PMC meiosis of this form
shows the synapsis conriguration 2n = 3x = 51 = 6I+ 9II+ 9III.
(4) Ser. 4 Kansuensis consisting of four species endemic to China, 2 of
which were cytologically studied, with the results: M. toringoides Hughes.
2n=34, 51, 68 and M. transitoria Schneid. 2n=34, 51. The karyotypes consist
of m (7—9 homologous chromosomes) and sm (7—9 homologous
chromosomes) as well as a few st chromosomes. The chromosomes range in
relative length from 9.44 to 4.01, with the ratios of the longest to the shortest
from 2.26 to 1.83, and the mean arm ratios from 1.87 to 1.72. In M. toringoides
four forms, 2x, 3x, 4x from Aba, and 3x from Barkang, in Sichuan Province,
are of 2B karyotypes while 2x from Year zhu in Sichuan Province is of 2A. A
mixploid with 3x, 4x in M. toringoides is found from Aba, with one smallest de-
leted chromosome at both levels much like that in M. xiaojinensis. PMC
meiosis of triploid M. toringoides from Barkang showed the synapsis
configurations of 10I+ 10II+ 7III. In M. transitoria 2x, 3x forms from Barkang,
Sichuan Province and Lüda, Liaoning Province belong to 2A, while 2x, 3x
forms from Aba, Sichuan Province both belong to 2B. PMC meiosis of triploid
form from Lüda showed 9I+ 9II+ 8III synapsis configurations.
(5) Ser. 5 Yunnanensis This series consists of four species endemic to China,
3 of which were cytologically studied with the results: M. ombrophila
Hand-Mazz. 2n=34; M. honanensis Rehd. 2n=34, and M. yunnanensis
Schneid. 2n= 34. The karyotypes consist of nine pairs of m, seven pairs of sm
and one pair of st chromosomes. The chromosomes range in relative length from
8.43 to 4.37, with the ratios of the longest to the shortest from 1.88 to 1.86,
and the mean arm ratios from 1.75 to 1.72. Therefore, the karyotypes belong to
2A. Two pairs of heterozygous chromosomes, 1st and 10th, are found carrying a
small satellite.
3 Sect. III Docyniopsis. This section consists of three endemic species to
China, 2 of which were studied with the results: M. formosana Kawak et
Koidz. 2n= 34 and M. melliana Rehd. 2n= 34. Both karyotypes consist of nine
pairs of m, seven pairs of sm and one pair of st chromosomes. The chromosomes
range in relative length from 8.90 to 4.48, with the ratios of the longest to the
shortest 1.85 and 1.83, and the mean arm ratios 1.67 and 1.66. Both karyotyoes
brlong to 2A.
As mentioned above, the karyotyoes of all the species examined are relatively
uniform, i.e. chromosomes small; chromosomes gradually decreasing in size from
the longest to the shortest; chromosomes mostly m and sm; and 1 —2 pairs of st
chromosomes always present. The karyotypes vary in the ratio of the longeat to
the shortest, the ratio of arms and the number of different chromosome types and
satellites. From cytological investigations and previous reports (Chen et al.,
1986; Liang, 1987), the cytology of the genus varies mainly in euploidy variation
and chromosome structure. Intraspecific polyploids and polyploid species make up
58. 1% in 24 species endemic to China reported. The karyotype analyse has
revealed the presence of three major types in Malus: 2A, 2B and 3B. The dele-
tion in some polyploid forms shows another structural variation. It is clear that
the three types are generally consistent with the separation of three sections of
Chinese Malus. According to Stebbins's (1971), the evolutionary trend of
karyotypes is from 2A to 2B and then 3B. The karyotypic data are in accordance
with the evidence from morphology (Langenfeld, 1971; Huckins, 1972) and
biochemistry (Williams, 1982). They both show that the section Docyniopsis and
series Yunnanenses are the most preimitive, while the series Pumilae, Baccatae of
the section Malus are the most advanced or derived. The most typical external
characters in Docyniopsis and the series Yunnanenses e.g. grit-cells in fruit, leaves
folded in bud, and the inflorescence according to Ponomarenko’s (1986) classifica-
tion, might be very primitive. The 2A karyotypes in the genus correlated with
many typically premitive characters. From the data mentioned above, we may
suppose that the series Sieboldianae and Kansuenses in the section Sorbomalus are
intermediate in evolution. In general, Southwest China is regarded as the probable
area of the origin of Malus, and the section Docyniopsis and the series
Yunnanenses as the relict primitive forms. This area is probably also the center
of differentiation.
A good deal of attention has been paid by horticulturists. A comparison of
the karyotypes in the series Pumilae shows that the diploid M. sieversii has 14 m
chromosomes, while the other diploids M. pumila, M. sp. and an alien
species M. sylvestris Mill. from Europe ( Liang, unpub.) have 12 m
chromosomes. Thus M. sieversii may be relatively primitive, although the ratio
of the longest to the shortest and the ratio of mean arm ratio are similar. The
present investigation confirms Ponomarenko’s view that the diploid M. sieversii
might be an ancestor of cultivated apple (M. pumila).
The wild polyploids of Malus were usually considered as allopolyploid
(Brown, 1976). Studies on the meiotic pairing of five triploids and one tetraploid
have revealed diverse types of polyploidy. M. hupehensis is a typical allotriploid
species, M. spectabilis, M. sieboldii, M. toringoides and M. transitoria are
segmental allotriploids, and M. xiaojinensis is almost an autotetraploid.
Apomixis in all known polyploid species of Malus is facultative. The princi-
pal adaptive advantage of facultative apomixis is to make sterile individuals
restore fertility, and also to maintain evolutionary potential. Therefore, it plays
an important role in evolution of Malus, but taxonomy of apomictical groups re-
mains highly controversial. Usually, the apomictical taxa are not treated as
species level (Stace, 1980; Hong, 1990). In Rehder’s (1940) system, only
M. hupehensis was recognized as species in apomictical taxa of Malus. We think
this treatment to be reasonable because M. hupehensis is an allotripoid and so far
its diplois form has not been found. New species M. xiaojinensis described
by Cheng and Jiang (1985) is nearly an autotetraploid, and its karyotype consti-
tution is extremely similar to the tetraploid form of M. toringoides from Aba ,
Sichuan Province, especially the remarkable similarities of the smallest deleted
chromosome, morphology and biochemistry (Li, 1990). We think, therefor, that
the triploid form of M. toringoides from Aba, Sichuan with 3x, 4x mixploidy
might be the maternal plant of M. xiaojinensis. For this reason, M. xiaojinensis
should be placed in apomictical polyploid complex of M. toringoides of the series
Kansuensis, and this is also in accordance with Rehder’s classfication.
